Gas liquid chromatography-mass spectrometry (GLC-MS), ion exchange chromatography, high pressure liquid chromatography (HPLC) and other separation procedures are used to isolate and measure the specific activities of the dopamine and norepinephrine (NE) metabolites, homovanillic acid (HVA), 3-methoxy, 4-hydroxphenylglycol (MHPG) and vanillyl mandelic acid (VWA) in the urine of controls and patients with orthostatic hypotension as well as with various neurologic and mental disorders. Deuterated D(-) MHPG has been injected intravenously into human controls quickly and as a slow, 3-hour infusion. Blood and plasma values for endogenous and deuterated MHPG and VMA have been analyzed to study the kinetics of conversion of MHPG to VMA and the rates of elimination of the compounds. 18O dopamine has been injected into several human controls and the relative enrichments of several dopamine and norepinephrine metabolites studied.